Microwave unit seal



. J. H. KLUCK Mano WAVE uui'r SEAL June 3, 1969 Sheet or-2 Filed Jan.11. 1967 6) .lllqlllll lllllllllllll lllll'lllllllllllllillllllllllllllllllIIIIIHIHIIIIIHH /7\\\\\\\\\\\\\\\\\\\\\\&\\\\\\\\\\\\\\\ June 3,1969 .1. HJKLUCK 3,448,232MICROWAVE UNIT SEAL I Sheet 2 of2 Filed Jan. 11,' 19a? I N VENTOR. MamaK4042 United States Patent 3,448,232 MICROWAVE UNIT SEAL James H. Kluck,Altadena, Califi, assignor to Hammtronics Systems, Inc., Pasadena,Calif., a corporation of Delaware Filed Jan. 11, 1967, Ser. No. 608,521Int. Cl. H05b 9/06 US. Cl. 219-1055 9 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to microwave units and more particularly tomicrowave units or electronic ovens having seals for preventing theleakage of radiation from the microwave unit proper.

In a microwave oven operating at frequencies above nine hundredmegacycles (900,000,000 cycles) it is necessary to provide an effectiveseal around the access door openings to prevent the leakage of thepropagating energy from the microwave chamber or oven. When thefrequency of the propagating energy has a wavelength that is smallerthan the principal dimensions of the door a significant amount of energycan escape through the gaps between the door and the chamber proper whenthese gaps are only a few thousand-tbs of an inch. Although the energyradiated from such a gap may be only a small fraction of the totalenergy directed into the microwave unit, it can readily be at a highenough level to be dangerous to personnel working in the vicinity of theunit. The leakage field will be particularly severe in cases of alightly loaded microwave unit where very high field strengths arepresent in the unit interior. The maximum tolerable field strength forhumans is generally considered to be .01 watts per square centimeter. Itcan be readily appreciated that only a small amount of leakage radiationcould produce field strengths of this magnitude or greater when amicrowave unit or oven is operated at power levels in the neighborhoodof one thousand watts or more, as is the usual case. It is thereforenecessary to provide a door closure which provides an effective seal tomicrowave energy for use in such units directly by making goodmetal-to-metal contact with the door frame around the entire perimeterof the access opening or door.

It has been obsreved that under the prolonged service normally expectedof a microwave unit or oven the contacting plate type of seal eventuallybecomes deformed and its sealing effectiveness is degraded. A moreeffective seal is provided by a contacting type seal used in conjunctionwith a quarter wave transmission line technique which reduces thevoltage appearing at the contact point of the door seal. This lattertype of seal, however, adds a complication to the apparatus and isfrequency sensitive so its effectiveness is reduced for operation atfrequencies slightly removed from the operating frequency for which thequarter wave section is designed. The quarter wave transmission linetechnique used alone is unsatisfactory since it requires precisetolerances on dimensions in order to be effective and is satisfactoryonly within a frequency band of about one percent (1%). This isconsiderably less than the frequency excursions encountered in practicewhich can be four percent (4%) or more within the Federal CommunicationsCommission (FCC) requirements for this type of equipment. Furthermorethe quarter wave choke design is only effective for the particular modeof propagation-for which it is designed and in the case of a largevolume electronic oven a number of, modes can be propagated in thevicinity of the gap which are not appreciably affected by the presenceof the choke and hence are not appreciably reduced by it. Othercontacting types of seals make use of braided wire gaskets which areeasily damaged, are readily contaminated by food particles, and aredifiicult to clean. Furthermore, both the contacting plate type and themetalized gasket type of door seal require considerable closing pressurein order to effect a good seal against the leakage of radiation.

The type of microwave unit under consideration in the presentapplication and towhich the present invention is applicable is disclosedin US. Patent 3,235,971 granted on Feb. 22, 1966 to the same assignee asthe present application. The microwave unit disclosed in theaforementioned patent is directed to a microwave unit or electronic ovenfor drying food pieces.

The present invention provides an improved microwave unit having adurable, easily cleaned seal against the leakage of harmful radiationwhich does not impede the opening and closing of the closure or door,for the microwave unit, without resorting to the use of a closing biaspressure on the closing member and yet maintains an effective sealagainst the escape or leakage of harmful radiation. The sealing means ofthe present invention is constructed and adapted to be substantiallyindependent of frequency over the allowable operating frequency range ofthis type of microwave unit as specified for the ISM bands by theFederal Communications Commission. For example, the frequency range maybe between 915 and 2450 megacycles. The sealing means of the presentinvention requires a minimum closing pressure and yet does not requireuniform contact between the cooperating parts in order to be effective.When the invention is employed with a pressure vessel, the seal will notinterfere with the conventional pressure seals such as conventionalO-rings or gaskets for sealing against external or internal pressures.

From a structural standpoint, the present invention cOmIprehends anelectrical conductive shell defining a microwave chamber adapted for thepropagation of microwave energy therein. The shell is provided with anaccess opening and a closure member or door for the shell to seal offthe access opening of the chamber to restrict the propagation ofradiation in the thus defined chamber. To provide the sealing means forthe chamber at the access opening the chamber and the closure member aredefined to include a plurality of coacting electrical conductiveelements defined to effectively short circuit any microwave energytending to leak from the chamber through the gap between the closuremember and the adjacent chamber wall. The electrical conductive elementsare constructed to be dimensionally defined with respect to theoperating microwave range of the unit and spaced apart based on thisoperating range whereby when the closure member and the chamber arearranged in a closed fashion both the vertical and parallel componentsrepresentative of the propagating electrical field are cut off therebypreventing the leakage of the propagating energy. The microwave unit ofthe present invention may further include a compressible gasket mountedto completely surround the series of conductive elements and of adimension to be engaged by a closure member in its normal closedposition.

These and other features of the present invention may be more fullyappreciated when considered in the light of the following specificationand drawings, in which:

'FIG. 1 is a perspective view of a microwave unit showing the closuremember in an open position and embodying the invention;

FIG. 2 is an enlarged, partial view of the elements comprising thesealing means arranged on one of the coacting surfaces illustrated inFIG. 1;

FIG. 3 is an enlarged, partial view showing the cooperating wall of themicrowave unit and the closure member in a closed and sealingrelationship;

FIG. 4 is a partial, top plan view of a modified microwave unitembodying the invention; and

FIG. 5 is an enlarged, partial view of the sealing means of the presentinvention for the microwave unit illustrated in FIG. 4 and shown in asealing relationship.

Now referring to the drawings, the microwave unit embodying the presentinvention will be examined in more detail. It should be understood atthe outset that the microwave unit 10 may be of any conventionalconstruction such as the microwave unit disclosed in the aforementionedU.S. Patent 3,235,971. Accordingly, it will be appreciated by thoseskilled in the art that any conventional source for providing themicrowave energy to be propagated within the microwave unit 10 propermay be provided for the purposes of this invention, the source ofmicrowave energy and means for propagating same within the unit 10 isnot illustrated to simplify the description of the present invention.The construction for these means illustrated in U.S. Patent 3,235,971may be employed, for example.

The microwave unit 10 is defined by a plurality of walls 11 arranged ina substantially rectangular configuration with each wall having itsinterior surfaces 11a of an electrical conducting material. The innerwalls 11a then define the microwave chamber 12 in which an article to bedried or heated is placed for subjection to the microwave energypropagating within the thus defined chamber. One of the walls, the frontwall 11, as illustrated in FIG. 1 is provided with an access opening 13to allow an operator access to the interior of the microwave chamber forplacing the article into the chamber or for removing the article fromthe chamber. As is conventional, the microwave chamber 12 is defined asa completely enclosed chamber by means of a closure or door 14illustrated as being hinged (not shown) to the front wall 11 at a pointidentified by the reference numeral 15 and dimensionally defined tocompletely cover the access opening 13. The inner surface 14 for theclosure member 14 is constructed of an electrically conductive material.It will be recognized by those skilled in the microwave art that theabove-described construction of the Inicrowave unit 10 is ofconventional construction.

An important feature of the present invention is the provision of thesealing means between the front wall 11 of the microwave unit 10 and theclosure member 14. For this purpose a sealing means is illustratedmounted on the front wall 11 as a plurality of electrical conductiveelements 16 spaced around the access opening 13. The conductive elements16 are mounted so that they protrude from the front face of the wall.Coacting with the elements 16 mounted on the front wall 11 are similarlydefined and constructed elements 17 mounted to protrude from theelectrically conductive surface or inner surface 14 of the closuremember 14. The conductive elements 16 and 17 are defined to interfitwith one another when the closure member 14 is placed in its normalclosed position and are electrically defined to effectively cut off boththe vertical and parallel components of the electrical field for themicrowave energy propagating in the chamber 12 as will be made moreevident immediately hereafter.

As is best illustrated in FIG. 2, the electrically conductive elements16 mounted on the front wall 11 of the unit are dimensionally definedwith a long and short dimension and with the elements 16 arranged withthe long dimension substantially perpendicular to the line defining theouter periphery of the access opening 13. The elements 16 are arrangedin a spaced-apart relationship and which distance is defined by theelectrical characteristics of the energy propagating in the chamber 12as will be noted hereinafter. The arrangement of the electricalconductive elements 16 in the corner of the wall 11 corresponding to thecorner of the access opening -13, as illustrated in FIG. 2, should benoted along with the spacing of the elements 16 a preselected distancefrom the access opening 13.

With the above-described structure in mind, the operating principles ofthe sealing means of the present invention will be examined. Theelectric field pattern of the energy tending to leak out through the gapbetween the front wall 11 and the closure surface 14a can be resolvedinto a vector component, E which is perpendicular to the gap and avector component, E which is parallel to the gap, as shown in FIG. 3.Both of these electric field vectors must be suppressed to eliminateenergy leakage through the gap. As is well known to those skilled in themicrowave art, as the distance between the conducting walls parallel tothe electric field vector is reduced, a point is reached at which thistransmission system will no longer support propagation and the system isconsidered to be at its cutoff point. In the parallel plate transmissionwhich is approximated by the microwave unit door gap, the cutoff pointoccurs when the wall spacing is equal to one-half the wavelength of theexciting energy, at which point the transmission of real energy ceases.The electric and magnetic fields still extend into the transmissionregion but diminish exponentially with distance so that at the exit endof this transmission system the radiation field excited by them isgreatl reduced. The magnitude of the fields extending into the cutoffregion is reduced by reducing the spacing between the conducting wallsparallel to the electric field vector. Likewise the magnitude of thefields appearing at the exit end of the transmission system is furtherreduced by increasing the length of the transmission path. In such acutoff waveguide system the attenuation is given reasonably accuratelyby the expression wherein A is the attenuation in decibels, db, 1 is thelength of the cutoff region and cl is the distance between the wallswhich are parallel to the electric field. This attenuation is almostwholly reactive so that the real power absorbed in the cutoff region isnegligible. Furthermore, it can be determined from the above equation,that since the frequency is not a factor thereof, the attenuation isindependent of frequency. These properties are very advantageous whenapplied to the microwave unit 10 where maximum efficiency is requiredand Where the frequency of operation may vary considerably.

An examination of FIG. 3 will reveal that the electric field vector, Edirected parallel to the gap is effectively suppressed by reducing thegap formed by conducting walls 11 and 14 so that they form a regionwhich is well below cutoff. For the highest frequency commonly used formicrowave units employed as electronic ovens, 2450 megacycles, thecutoff point, occurs for a spacing of 2.41 inches. Therefore, it shouldbe apparent that under normal conditions that when the door 14 is closedthe gap formed between the door 14 and the front wall 11 will be smalland substantially below cutoff for the electric field polarized parallelto the gap. The gap spacing and the length of the cutoff region may thenbe adjusted to provide the discrete attenuation and suppression of thisfield in accordance with the cutoff attenuation specified above. Forexample, applying the above principles, a gap spacing of 0.20 inch forfor a length of 0.50 inch will provide '68 decibels of leakagesuppression.

The electric field polarized perpendicularly to the gap in the absenceof the elements 16 and 17 is not appreciably suppressed since the widthof the gap for the usual microwave unit is so large that a below cutoffsituation is not attained. Hence, it has been established that theinterlocking elements 16 and 17 approximate the necessary walls parallelto the electric field to produce a cutoff condition. A true cutoffcondition could only be achieved if the elements 16 and 17 made a solidconnection to the opposite wall. It has been determined, however, that aconsiderable gap may exist, while still pro viding a high degree ofsuppression. It is believed that the effect of the gap is to increasethe width of the cutoff dimension to the width between adjacentconductive elements 16 on the same mounting surface rather than thewidth between adjacent elements when the closure member 14 is closed(the distance between an element 17 and an element 16 when the door 14is in the closed position).

The cutoff equation specified above may be applied to the configurationof the conducting elements 16 and 17 to obtain dimensional data usingthe spacing between adjacent elements on the same mounting surface asthe distance d in the equation. For example, using elements having athickness of 0.040 inch, a separation of 0.25 inch between centers, anda transmission length of 0.5 inch would provide a suppression of 65decibels.

In accordance with the teaching of this invention, a microwave unit hasbeen constructed employing the conductive elements 16 and 17. For thispurpose, elements 16 were 0.50 inch long, .040 inch thick, and extended0.19 inch above their mounting surface, and were spaced 0.25 inchbetween centers. The elements 17 mounted on the door 14 were identicallydimensionally defined. One set of the elements 16 or 17 is displaced asshown in FIG. 3 so that the elements 17 on the door 14 bisected thespace between the elements 16 on the front wall 11. With such amicrowave unit 10, microwave energy was supplied to the chamber 12 at apower level of 1400 watts and which chamber was loaded with 1400 gramsof water. Under these operating conditions, a maximum total fieldstrength of 1 milliwatt per square centimeter was measured close to thesealed ggap. This is well below the tolerance level of milliwatts persquare centimeter and is approximately the same level as the radiationintensity measured from a commercial oven employing a thin platecontacting closure in conjunction with a quarter wave choke recess. Ithas also been determined as a result of such operation that the leakagewas not affected by small changes in the gap between the door 14 and thefront wall 11. To this end, a gap of between 0.03 and 0.13 inch betweenthe elements 16 and 17 and the opposite surface could exist withoutnoticeably affecting the level of leakage energy.

Another important aspect of the invention that has been determined isthat improved sealing action results when the elements 16 are set backfrom the access opening 13 a small distance s. It has been found thatthis arrangement of the elements 16 in conjunction with a similararrangement of the elements 17 provides added suppression to leakageenergy polarized parallel to the gap. In microwave units 10, then,operating at power levels in excess of 1000 watts, it is preferred tospace the elements 16, one-fourth inch away from the access opening 13(s=% inch) to achieve the benefit of this additional leakagesuppression.

In FIGS. 4 and 5 another embodiment of the microwave unit 10 isillustrated embodying the invention. This embodiment is essentially thesame as discussed hereinabove except that the access opening 13 is of acircular construction and the door 14' is also defined as circular ingeometry. The sealing elements 16 and 17, however, are similarly definedand arranged around the access opening 13 and on the inner conductiveface of the door 14'; see FIG. 5 in particular.

When it is desired to provide an air or pressure seal in addition to themicrowave energy seal, a conventional compressible gasket may beprovided around the external perimeter of the door seal. One suchcompressible gasket is illustrate-d in FIG. 4 and is identified by thereference character 20. The non-critical nature of the allowable gapbetween the elements 16 and 17 and their opposing walls facilitates theuse of such a gasket. The gasket 20 may be advantageously constructed toserve not only as an air seal but also can be constructed of aconductive or microwave absorbing material to provide furthersuppression of the leakage of energy.

What is claimed is:

1. In a microwave unit including a microwave chamber adapted for thepropagation of microwave energy therein and having an opening in atleast one wall therein to provide access to the interior of the chamber,the wall providing the access opening being constructed and defined apreselected distance from the outer periphery of the wall to provide asealing surface, a plurality of solid electrically conductive elementsmounted on the thus defined sealing surface at preselected spaced apartlocations and completely encircling the access openings, and a closuremember for the access opening adapted for completing the microwavechamber, the closure member having a plurality of solid electricallyconductive elements mounted adjacent and around its outer periphery atpreselected spaced apart locations to interfit with the similarlydefined conductive elements on the sealing surface when the closuremember is applied to close the access opening whereby the interfittedelements coact to effectively short circuit the components of the fieldpattern of any microwave energy tending to leak from the chamber andthereby effectively seal the microwave energy within the chamber.

2. In a microwave unit as defined in claim 1 wherein the electricallyconductive elements on the sealing surface and the closure member aredimensionally defined with a long and a short dimension and the elementsare arranged with the long dimension being substantially perpendicu-larto the line defining the outer periphery of the access opening.

3. In a microwave unit as defined in claim 2 wherein the electricallyconductive elements on the closure member are spaced to interfit withthe elements on the sealing surface at a point substantially centrallyof the distance between adjacent elements on the sealing surface.

4. In a microwave unit as defined in claim 1 wherein the conductiveelements interfit without contacting one another or the adjacentsurface.

5. In a microwave unit as defined in claim 1 including a compressiblegasket mounted on the chamber to completely surround the conductiveelements and of a dimension to be firmly engaged by the closure memberin its normal closed position.

6. In a microwave unit as defined in claim 5 wherein the compressiblegasket is constructed of an electrical conductive material.

7. In a microwave unit as defined in claim 6 wherein the compressiblegasket is constructed of a material capable of absorbing microwaveenergy.

8. In a microwave unit as defined in claim 1 wherein the electricallyconductive elements are mounted on the sealing surface a preselecteddistance from the outer periphery of the access opening.

9. In a microwave unit including an electrically conductive shelldefining a microwave chamber adapted for the propagation of microwaveenergy therein, the shell having an access opening therein, and aclosure member for the shell to seal off the access opening of thechamber to restrict the propagation of the microwave energy within thethus defined chamber, said chamber and said clo sure member including aplurality of solid electrically conductive elements spaced apart andinterfitted to coact with each other to effectively short circuit anymicrowave energy tending to leak from the chamber through the gapbetween the closure member and the adjacent chamber wall thereby sealingoff the chamber, the electrically conductive elements beingdimensionally defined and spaced apart to provide an electrical cutoffof the components of 7 8 the electrical field pattern that are bothperpendicular and 3,196,242 7/1965 De Vries et al. 219-10.55 parallel tothe gap between the closure member and the 3,219,747 11/1965 McAdams21910.55 chamber and thereby provide a seal independent of the 93,260,832 7/1966 Johnson 219-10.55

frequency of the propagating energy within the chamber.

RICHARD M. WOOD, Primary Examiner.

5 References Cited L. H. BENDER, Assistant Examiner. UNITED STATESPATENTS 2,956,143 10/1960 Schall 219 10.s5 1 10 US 2,958,754 11/1960Hahn 219 10.55 2

3,182,164 5/1965 Ironfield 21910.55 10

